My 14 year old dachsaunds jaw spasms open when he's eating, why?

When he's really chewing hard, his mouth spasms open and he lets the food fall out of his mouth and just openly chews with his tongue hanging out of the side of his mouth, its dishearting and kills me to see. I saw him do this for the first time last night when he wasn't eating - just a spasm. Why?!